Multi-mode television receivers capable of reception and display of television signals of different transmission standards are known. Hinn, for example, describes a two-mode receiver for signals of PAL or SECAM format in U.S. Pat. No. 4,309,719. An example of three mode receiver processing for NTSC, PAL-M and PAL-N is described by Lim in U.S. Pat. No. 5,119,177.
It is conventional practice in multi-standard receivers to alter the display scan rate to match that of the received signal and to transcode the received chrominance signal to a common color standard. Advantageously, this simplifies later color processing steps such as hue and saturation control, flesh-tone correction and the like.
The use of a chrominance signal transcoder for converting received color signals to a common standard may present a problem with regard to providing proper registration between the received luminance and chrominance components for different transmission standards. Specifically, transcoding of the chrominance signal introduces a processing delay. To maintain proper registration of the luminance and chrominance signals in different operating modes, it is desirable that additional delay be inserted in the luminance signal path when the received signal requires transcoding to compensate for the transcoding delay imparted to the chrominance signal by the transcoder. If some form of delay compensation is not provided, visual artifacts (e.g., color errors) may be produced when switching between video signals of different transmission standards.
The problem of delay compensation in multi-mode receivers is recognized in the above-mentioned Hinn patent where it is noted that one possible solution to the problem would be to provide a compensating delay of a fixed "compromise" value (e.g., lying midway between the delays required in each video transmission standard). Hinn points out, however, that in cases where the delay difference is relatively large (e.g., 500 nano-seconds) that reliance on a compensating delay of a fixed "compromise" value may provide unsatisfactory results.
A solution to the problem of color registration in multistandard receivers, proposed by Hinn, is to provide a switchable delay in the luminance signal channel. Advantageously, this completely avoids any potential errors due to the use of fixed "compromise" delay compensation values and provides exact delay compensation for any transmission format of the video signal. More specifically, in the Hinn system, a switch adds extra delay to the luminance signal path when receiving signals that require chrominance transcoding from one standard to another (e.g., SECAM to PAL in the Hinn system). For signals of a format that does not require transcoding (e.g.,
in the Hinn system), the delay is switched out of the luminance path. Advantageously, this method of delay compensation avoids the inaccuracies of "compromise" delays and ensures that the luminance and chrominance signals receive equal delays regardless of the format (PAL or SECAM) of the received signal.